The OSI model is the part of the Open Systems Interconnection project at the International Organisation for Standardisation.

The OSI model characterizes and provides a standard set of protocols for communication functions of a telecommunication or network system regardless to its underlying architecture.

It provides interoperability and helps in easy troubleshooting of the network.
The OSI model is not a protocol, it is a network architecture that is flexible, robust and inter-operable.

It is divided into seven different layers:

  • Physical Layer
  • Data link Layer
  • Network Layer
  • Transport Layer
  • Session Layer
  • Presentation Layer
  • Application Layer

Lets look into each layer one by one,

Layer 7: Application layer

  • The application layer is the layer close to the user, which means both the application layer and the user interact with the software application.
  • It acts as a interface between application and user.
  • Application-layer functions include identifying communication partners, determining resource availability, and synchronising communication.

Layer 6: Presentation layer

  • The presentation layer handles the delivery and formatting of information. this layer presents data for the application or the network.
  • It converts data into the standard format.
  • It performs functions such as encryption, decryption, data compression, data decompression and character encoding.
  • Protocols used are eXternal Data Representation (XDR), Network Data Representation (NDR), X.25 Packet Assembler/Dis-assembler Protocol (PAD), Lightweight Presentation Protocol (LPP), Internet key exchange (IKE)

Layer 5: Session layer

  • Its main function is to manage communication sessions – continuous exchange of information.
  • It provides the mechanism for opening, closing and managing a session between end-user application processes.
  • It creates a session ID for each application and keeps the data separate for each.
  • In case of a connection loss this protocol will try to recover the connection.
  • If a connection is not used for a long duration, then session-layer protocol will close or re-open it.
  • It provides either full duplex or half-duplex operation and provides synchronisation points in the messages exchanged.
  • Protocols used are ADSP, ASP, H.245, ISO-SP, iSNS, L2F, L2TP, NetBIOS, PAP, PPTP, RPC, RTCP, SMPP, SCP, SOCKS, ZIP, SDP, SIP.

Layer 4: Transport layer

  • Transport layer provides functions and procedures for transferring data to the destination.
  • Its the bridge between hardware and software layer. The heart of the OSI layer.
  • The transport layer controls link through flow control, segmentation/de-segmentation, and error control.
  • This layer can keep track of the segments and re-transmit those that fail delivery.
  • The transport layer also provides the acknowledgement signal on successful transmission of data and sends the next data if no errors occurred.
  • It creates segments of the data dividing them into smaller layer.
  • Responsible for process to process delivery.
  • Protocols used are TCP and UDP.

Layer 3: Network Layer

  • The Network layer is responsible for the delivery of individual packets from the source host to destination.
  • It is used to communicate between different network.
  • This layer uses logical addressing (adds IP address in the header).
  • It decides the path in which the information must flow.
  • The main functions are Routing and forwarding data, logical addressing, error detection, sequencing of data and flow control.
  • Data is in the form of packets.
  • Protocols used are RIP, OSPF, BGP, EIGRP, Apple Talk
  • Devices: Router, Firewalls, L3 switch

Layer 2: DataLink Layer

  • This layer is responsible for moving frames from one node to another.
  • It is responsible for communication with the Local Area Network.
  • Data shared is in the form of Frames.
  • Responsibility of this layer are framing, physical addressing, flow control, error control, access control.
  • Physical addressing – also called as MAC addressing or Layer 2 addressing.
  • It is divided into two sublayers : Logical Link Control (LLC) and Media Access Control (MAC).
  • MAC address is a 48 bit hexadecimal address.
  • Protocols used are ARP, RARP, ICMP, IGMP, HDLC, PPP.
  • Devices: NIC, Switch, Bridge.

Layer 1: Physical layer

  • The Physical layer is responsible for movement of individual bits from one hop to another.
  • It coordinates the function required to carry a bit stream over a physical medium.
  • Main functions are modification of data into digital signals, establish and termination of connection, effectively share resources among multiple users.
  • Devices: NIC, HUB, Repeaters and Modem.

Remembering the OSI Model 7 layers – 8 mnemonic tricks:

From Application to Physical:

All People Seem To Need Data Processing
All Pros Search Top Notch Donut Places
A Penguin Said That Nobody Drinks Pepsi
A Priest Saw Two Nuns Doing Pushups

From Physical to Application:

Please Do Not Throw Sausage Pizza Away
Pew! Dead Ninja Turtles Smell Particularly Awful
People Don’t Need To See Paula Abdul
Pete Doesn’t Need To Sell Pickles Anymore

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